1. Field of the Invention
The present invention relates to a thermal flow meter that measures a flow rate by placing a heat-generating resistor in a fluid to be measured, and more particularly relates to a thermal flow meter suitable for measuring an intake air flow rate or an exhaust gas flow rate of an internal combustion engine in an automobile.
2. Description of the Related Art
Thermal air flow meters capable of directly measuring a mass flow rate have become mainstream, as flow rate sensors that detect an intake air amount of an internal combustion engine in an automobile or the like.
In recent years, a thermal flow meter in which sensor elements are formed on a semiconductor substrate such as a silicon (Si) substrate, by using the micro machine technology. In such a semiconductor type thermal flow rate sensor, a hollow section is made by cutting out a part of the semiconductor substrate in a rectangular shape, and a heat-generating resistor is formed on an electrical insulating film having a thickness of a few micrometers that is formed in the hollow section. The heat-generating resistor has a micro size of several hundreds micrometers and is formed in a thin film shape, so that the thermal capacity thereof is low, thereby achieving fast response and low power consumption. Furthermore, a forward flow and a reverse flow can be distinguished by using a temperature difference method in which temperature sensors (temperature measuring resistors) are formed at the upstream and downstream sides of a vicinity of the heat-generating resistor, respectively, and a flow rate is detected based on a temperature difference between the temperatures at the upstream and downstream sides of the heat-generating resistor.
Consider a case where such an air flow meter is mounted to an internal combustion engine in an automobile and the like, and is used under harsh environment conditions. In such case, the air flow meter is exposed to harsh temperature conditions depending on a season, an area in which the air flow meter is used, and heat generated by an engine. Thus, a flow rate sensor should be capable of detecting a flow rate with high accuracy even under conditions where the temperature changes between −40° C. to +125° C. JP Patent Application Publication No. 2000-314645 discloses a flow rate sensor as a related art applicable to such temperature conditions.
The technology disclosed in JP Patent Application Publication No. 2000-314645 is a circuit configuration in which a heating temperature of a heat-generating resistor is determined based on a resistance value of a fixed resistance having small variations in resistance value caused by temperature. Accordingly, the resistance value of the fixed resistance does not vary with outside temperature changes, so that a temperature difference between the temperature in the heat-generating resistor and the temperature outside the heat-generating resistor can be held constant, thereby reducing variations of detection sensitivity. In addition, a resistance balance is adjusted by a variable resistance in order to deal with fluctuations of the resistance values of temperature measuring resistors or fixed resistances due to the production process.